As is known, systems for energy scavenging or energy harvesting from intermittent environmental energy sources (which supply, that is, energy in an irregular way) arouse and continue to arouse considerable interest in a wide range of fields of technology. Typically, energy-scavenging systems are configured to harvest, store, and transfer energy generated by mechanical sources to a generic load of an electrical type.
Low-frequency vibrations, such as, for example, mechanical vibrations of disturbance in systems with moving parts, may be a valid source of energy. The mechanical energy is converted by one or more purposely provided transducers (for example, piezoelectric or electromagnetic devices) into electrical energy, which can be used for supplying an electrical load. In this way, the electrical load does not require batteries or other supply systems that are cumbersome and have a poor resistance to mechanical stresses.
FIG. 1 is a schematic illustration by means of functional blocks of an energy-scavenging system of a known type.
The energy-scavenging system 1 of FIG. 1 comprises: a transducer 2, for example of an electromagnetic type, configured to convert the mechanical energy of the environmental mechanical vibrations into electrical energy, for example into AC voltages or voltages randomly variable in time; a scavenging interface 4, for example comprising a diode-bridge rectifier circuit (also known as Graetz bridge), configured for receiving at input the AC signal generated by the transducer 2 and for supplying at output a DC signal, or in general a slowly variable signal that can be considered as a DC signal, for charging a capacitor 5 connected on the output of the rectifier circuit 4; and a DC-DC converter 6, connected to the capacitor 5 to receive at input the electrical energy stored by the capacitor 5 and supply it to an electrical load 8. The capacitor 5 functions as an element for storage of energy, which is made available, when required, to the electrical load 8 for operation of the latter.
The global efficiency ηTOT of the energy-scavenging system 1 is given byηTOT=ηTRANSD·ηSCAV·ηDCDC  (1)
where: ηTRANSD is the efficiency of the transducer 2, indicating the amount of power available in the environment that is effectively converted, by the transducer 2, into electric power; is the efficiency of the scavenging interface 4, indicating the power dissipated by the scavenging interface 4 and the factor ηCOUPLE of impedance matching between the transducer 2 and the scavenging interface 4; and ηDCDC is the efficiency of the DC-DC converter 6.
As is known, in order to supply to the load the maximum power available, the impedance of the load should be the same as that of the source. As shown in FIG. 2, the transducer can be represented schematically, in this context, as a voltage generator 3 provided with a resistance RS of its own. The maximum power PTRANSDMAX that the transducer 2 can supply at output can be defined asPTRANSDMAX=VTRANSD2/4RS if RLOAD=RS  (2)
where: VTRANSD is the voltage supplied by the equivalent voltage generator; and RLOAD is the equivalent electrical resistance on the output of the transducer 2 (or, likewise, the resistance seen at input to the scavenging interface 4), which takes into due account the equivalent resistance of the scavenging interface 4, of the DC-DC converter 6, and of the load 8.
On account of the impedance mismatch (RLOAD≠RS), the power at input to the scavenging interface 4 is lower than the maximum power available PTRANSDMAX.
The power PSCAV transferred on the capacitor 5 is a fraction of the power recovered by the interface, and is given byPSCAV=ηTRANSD·ηSCAV·PTRANSDMAX  (3)
The power required of the DC-DC converter 6 for supplying the electrical load 8 is given byPLOAD=PDCDC·ηDCDC  (4)where PDCDC is the power received at input by the DC-DC converter 8, in this case coinciding with PSCAV, and PLOAD is the power required by the electrical load.
The efficiency of the system 1 of FIG. 1 is markedly dependent upon the signal generated by the transducer 2. The efficiency drops rapidly to the zero value (i.e., the system 1 is unable to harvest environmental energy) if the amplitude of the signal of the transducer (signal VTRANSD) assumes a value lower, in absolute value, than VOUT+2VTH—D, where VOUT is the voltage stored on the capacitor 5 and VTH—D is the threshold voltage of the diodes that form the scavenging interface 4. As a consequence of this, the maximum energy that can be stored in the capacitor 5 is limited to the value Emax=0.5·COUT·(VTRANSDMAX−2VTH—D)2. If the amplitude of the signal VTRANSD of the transducer 2 is lower than twice the threshold voltage VTH—D of the diodes of the rectifier of the scavenging interface 4 (i.e., VTRANSD<2VTH—D), no energy is harvested from the environment, and the load is not supplied.